1. Field of the Invention
The invention relates to belt tensioning devices, and in particular to mechanical spring-actuated belt tensioning devices for use with the endless drive belts of the drive systems for vehicle accessories. More particularly, the invention relates to a relatively simple and compact belt tensioner operated by a torsional spring which maintains a predetermined tensioning force on the endless drive belt and which has improved shock-absorbing damping characteristics.
2. Description of the Prior Art
There is the trend today in the automobile industry to operate the various vehicle accessories, such as the power steering pump, water and air pumps, air conditioning and alternator, by a single endless belt driven by a pulley connected to the engine crankshaft. This system is referred to as a "serpentine" drive belt system. To ensure optimum operating efficiency for these various accessories, it is necessary that the drive belt be maintained at a predetermined tension to assure efficient performance of the accessories as well as satisfactory service life for the belt. Due to the relatively greater length of the single drive belt which replaces the heretofore plurality of smaller belts, there is a greater tendency for the belt to stretch which will affect the operating characteristics of the driven accessories. Therefore, it is desirable that a belt tensioning device be used for these endless belts to provide reliable service over an extended period of time and to maintain a constant amount of tension thereon regardless of the amount of belt stretch.
Numerous devices have been proposed and used to accomplish this purpose. One type of tensioner uses a bushing formed of an elastomeric material which is placed in compression by some mechanical means for continuously exerting a tensioning force on the belt. Examples of these constructions are shown in U.S. Pat. Nos. 3,975,965 and 4,144,772. These tensioners which use an elastomeric material as the tensioning means have the disadvantages in that the high load rate which they exert on the belt results in the rapid loss of tensioning as the belt stretches, and this load rate limits the stroke of the belt-engaged idler pulley to a shorter distance than desired. Also, sudden acceleration and deceleration of the drive belt can cause a whipping action to occur which creates a time lag before full damping is achieved.
Numerous other types of belt tensioning devices use compression or tension springs for applying and maintaining the tensioning force on a belt-engaging idler pulley or chain-engaging sprocket. Some examples of these types of constructions are shown in U.S. Pat. Nos. 2,703,019, 2,893,255, 3,413,866, 3,483,763, 3,631,734, 3,768,324, 3,812,733 and 3,965,768. Other various spring-actuated devices use the biasing force of a tension or compression spring in combination with hydraulic-actuated members for regulating the amount of tensioning force applied to the belt, depending on whether the engine is running or shut off. Examples of these combination hydraulic and spring-actuated tensioning devices are shown in U.S. Pat. Nos. 2,051,488, 3,142,193, and 4,077,272.
Due to the relatively large number of different vehicles and the different drive belt and accessory arrangements therefor, it is difficult to provide a tensioner construction which can be adapted for use in nearly all of these various installations. Also, it is desirable that such a belt tensioner can exert the necessary tensioning force and have a compact, rugged and inexpensive construction to minimize both original equipment cost and installation cost, and which reduces or eliminates maintenance.
The above-listed examples of spring-actuated belt tensioners use a coil spring which is placed in compression or tension for applying the belt tensioning force. It is difficult to achieve high tensioning forces with springs which are placed in tension or compression in a relatively small compact unit due to the size of the springs required to develop relatively large tensioning forces.
Other known tensioner constructions, such as shown in U.S. Pat. Nos. 3,136,170, 3,483,763, 3,834,246, 3,924,483 and 4,285,676, use torsional springs for pivotally moving an idler pulley or sprocket into tensioning engagement with a belt or chain, or for pivotally moving one of the vehicle accessories to achieve a desired tensioning force.
When a spring is used to apply the tensioning force, it is desirable that the belt tensioner be provided with some type of damping means to prevent excessive oscillation from occurring in the spring tensioning member. The damping means also will absorb sudden shocks to prevent a whipping action from occurring in the tensioner and drive belt, such as upon sudden acceleration and deceleration of the vehicle. This damping action is especially critical when a coil spring is used for applying the belt tensioning force since coil springs are highly susceptible to develop natural oscillating frequencies when the counter force which is exerted thereon by the belt fluctuates during acceleration and deceleration of the vehicle. However, such oscillating action also affects the efficiency of the tensioning force when applied to the belt by a torsional coil spring and will reduce belt life. These oscillating forces are even more pronounced in diesel engine-driven vehicles which are becoming more popular for the mass-produced passenger vehicle market.
Various damping devices have been used with belt tensioners to eliminate or reduce this problem of coil spring oscillation. One type of construction uses a hydraulic fluid as the damping means, such as shown in U.S. Pat. Nos. 2,893,255, 3,964,311, and 3,986,407. U.S. Pat. No. 3,710,634 shows a belt tensioner which uses an eccentrically mounted mechanical pinion and rack arrangement which is spring biased by a leaf spring for absorbing an excessive amount of shock as opposed to providing a damping action for a spring-biased belt tensioning plunger.
Therefore, the need has existed for a belt tensioner construction which is of a simple, rugged and compact design, which uses a coil torsional spring to achieve the belt tensioning force by directly engaging the belt with an idler pulley which is biased into belt tensioning engagement by the force of the torsional spring, and which is provided with damping means to reduce the natural oscillating frequencies that occur in coil springs.
There is no known belt tensioning construction of which I am aware which imparts a generally constant predetermined tensioning force on an endless drive belt by moving an idler pulley into tensioning engagement with the belt by the use of a coil torsional spring which is mounted in a telescopic relationship within a compact, rugged housing, and which has inexpensive damping means to reduce harmful oscillations and provide a shockabsorbing effect on the belt tensioner.